1. Field of the Invention
The invention concerns hot-melt adhesives and, more specifically, compositions of ethylene/vinyl acetate and tackifying resin. The invention is particularly concerned with the need for an adhesive which can be applied as a thin layer that remains tacky for a sufficiently long time at room temperature to permit a creep-resistant bond to be made merely by applying ordinary hand pressure.
2. Description of the Related Art
U.K. Pat. Spec. No. 1,233,197 (Cheetham et al.) which was published May 26, 1971 says: "Known hot-melt adhesives for bonding paper stocks are 100% solids systems consisting of, for example, a mixture of an ethylene/vinyl acetate copolymer, a resin and a wax; these adhesives are applied in the molten state and revert to a reversible solid, usually non-tacky, state on cooling . . . although there are some hot-melt adhesives which exhibit a degree of cold tack so that they can be cold bonded; these adhesives are usually wax-free or have a low wax content. It has been found, however, that bonds between paper substrates formed with these adhesives exhibit unacceptable creep or part completely at temperatures of about 50.degree. C. Packs made under such adhesives fail under tropical storage conditions"(page 1, lines 15-38). The Cheetham specification says that both "high cold-tack and resistance to cold-flow at 50.degree. C." can be attained by blending a glycerol ester of highly hydrogenated rosin and a hydrogenated methyl ester of rosin with the ethylene/vinyl acetate. In Example 1, such a blend was applied at 120.degree.-140.degree. C. in thin layers to paper pack blanks which were formed and sealed into packets under a pressure of approximately 7 p.s.i., with a delay as long as 16.8 seconds between the times of applying the adhesive and sealing a packet. The bonds were said to have no tendency to creep or burst open at 50.degree. C., but the forces are not disclosed and apparently were quite small.
By 1980, as reported in Duncan et al., "EVA and EVA Copolymers for Hot Melt PSA's," Adhesives Age, March 1980, pp 37-41:Ethylene/vinyl acetate (EVA) copolymers are well established as the base polymer for hot melt package sealing adhesives. EVA's can be formulated to give a broad range of properties over a wide range of temperatures. . . . The versatility of the 18 to 33 percent vinyl acetate EVA's is attributed to the polymer's dual nature. These EVA copolymers contain a balance of crystalline and non-crystalline regions, which allow them to accept a wide variety of modifiers. . . .
"The crystallinity, which is proportional to the ethylene content, contributes strength and heat resistance. The crystalline regions are compatible with paraffin and microcrystalline waxes. Waxes are used to lower viscosity, adjust open time, and reduce cost."
"The amorphous regions, which result from the presence of vinyl acetate, contribute flexibility and adhesion. . . . At about 45 wt. percent, the polymer becomes completely amorphous. . . . Pressure-sensitive tack is obtained by improving compatibility and reducing polymer crystallinity."
Although some EVA-based adhesives exhibit permanent pressure-sensitivity (as indicated in the Duncan publication), it is believed that no known EVA-based adhesive can be spread onto an unheated substrate as a thin layer about 50 .mu.m in thickness that (1) remains tacky for at least 5 seconds at room temperature and (2) then forms creep-resistant bonds without applying heat or more than hand pressure.
By "tacky" is meant that when contacted with ordinary typing paper under the force of one pass of a 2.2 kg roler, an immediate effort to remove the paper tears the paper. In other words, a 50 layer of adhesive remains sufficiently tacky to tear paper.
Of prior hot-melt adhesives, that which is believed to come the closest to achieving the above combination of properties (1) and (2) is the subject of U.S. Pat. No. 4,613,632 (Aliani et al.), namely, an EVA-based hot-melt adhesive including a resin that functions as a tackifier and also assists wetting. The Aliani adhesive can also contain a wax but preferably is wax-free. The EVA has a vinyl acetate (VA) content of from 11 to 40% by weight and a melt index of from 700 to 10,000 in a wax-free hot-melt system. The Aliani patent makes its test bonds at elevated temperatures, e.g., at 100-150.degree. C. (col. 12, lines 9-11), and at 90.degree. C. (col 14, line 48) and says nothing to indicate whether a thin layer of its adhesive might be tacky at room temperature. However, the Aliani patent say that its adhesive has "open time," and sometimes refers to "open time at 180.degree. C. "(col. 10, line 40) but without further explanation.
Our testing of EVA-based hot-melt adhesives similar to two of the Aliani (one at 27.5% VA content and melt index of 2500 and another at 14% VA content and melt index of 2500 ) shows that when a 3.2 mm bead is extruded at 130.degree. C. the bead retains sufficient heat in a room temperature environment to form a bond for up to two minutes. On the other hand, when the latter Aliani composition was spread onto an unheated substrate as a thin layer of about 50 .mu.m, the layer became nontacky almost instantaneously. When the former Aliani composition was so spread it remained marginally tacky for about 5 seconds but did not wet well enough to form a creep-resistant bond (reported in detail below as comparative Example C-1).
Hot-melt adhesives often are dispensed from a gun-like device such as illustrated in U.S. Pat. No. 4,621,748 (Dziki) which receives a rod of adhesive that is formed with teeth by which the adhesive is mechanically driven into the heated well of the gun-like device. At ordinary room temperatures, the teeth of the rod of adhesive must have sufficient strength to withstand the driving force. A similar dispenser is illustrated in U.S. Pat. No. 4,457,457 (Dziki), but uses a smooth-surfaced rod of adhesive that is driven into the heated well pneumatically. Hence, its rod of adhesive does not need to be as strong as a toothed rod, but it should have sufficient rigidity not to buckle under the pneumatic force.
Hot-melt adhesives are sometimes marketed as nontacky sticks which, when rubbed against a substrate, deposit a thin layer of adhesive that is tacky by virtue of heat generated by the rubbing. Those thin layers can remain tacky for a sufficient period of time to permit bonds to be made under ordinary hand pressure at room temperature. U.S. Pat. Nos. 4,066,600 (Pletcher) and No. 3,539,481 (Parker) concern stick adhesives but do not suggest the use of EVA-based adhesives.